The present invention generally relates to a transmission power control system used in a mobile communication system operable by a code division multiplex access system (simply referred to as a “CDMA” system hereinafter). More specifically, the present invention is directed to a transmission power control system capable of avoiding an occurrence of a so-called “call drop” phenomenon.
Since very recent electronic communication techniques are greatly progressed, mobile communication systems such as automobile telephone and portable telephone are rapidly popularized. In connection with this progress in the electronic communication techniques, the mobile communication systems are gradually transfixed from the time division multiple access (TDMA) system to the code division multiple access (CDMA) system.
Generally speaking, a CDMA type cellular mobile communication system owns the below-mentioned advantages.
(1) This cellular mobile communication system owns a high durability with respect to interference such as radio interference and radio disturbances.
(2) Since power spectrum density is low, interference given to other communication channels becomes small.
(3) Since power spectrum density is low, superior secrecies can be established.
(4) The superior secrecies can be established by employing spread codes.
(5) The multiple access can be carried out by using different spread codes.
(6) The mobile communications can be carried out under over load conditions.
However, while the CDMA type cellular mobile communication system can have the above-explained various merits, this cellular mobile communication system should own various problems. Among these problems, there is an interference problem. As a typical interference problem, a so-called “near-to-far” problem may occur. This “near-to-far” problem implies such a phenomenon that while a communication is established between a base station and a desirable mobile station located far from this base station, another interference mobile station located in the vicinity of this base station may give large interference to the desirable mobile station under communication. This “near-to-far” problem may occur not only in the CDMA type mobile communication systems, but also other conventional mobile communication systems. That is, this “near-to-far” problem is known as channel-to-channel interference (same channel interference/adjoining channel interference). In particular, since a large number of mobile stations commonly use the same frequency in the CDMA system, this interference problem may become serious problems.
To solve such an interference problem, various sorts of transmission power control units are provided in order to reduce the interference amounts in the CDMA type mobile communication systems. As these transmission power control units, there are two types of transmission power control units, namely a reverse-link (up-link direction) transmission power control unit operable for the mobile station to the base station, a forward-link (down-link direction) transmission power control unit operable for the base station to the mobile station.
Reception electric field strength measurement is carried out by a reception unit of the base station in the up-link transmission power control unit, and the base station measures the electric field strength of a radio signal transmitted from the mobile station. As a result of this field strength measurement, when the received electric field strength is higher than a predetermined threshold value, the base station instructs the mobile station to reduce the transmission power by using the power control bit. On the other hand, when the received electric field strength is lower than the predetermined threshold value, the base station instructs the mobile station to increase the transmission power by using the power control bit.
On the other hand, the received electric field strength measurement is carried out by the reception unit of the mobile station in the down-link direction transmission power control unit. The mobile station measures the received electric field strength of the information signal which is broadcasted from the base station. The mobile station produces the transmission power control information based upon the measured reception electric field strength, and then sends the produced transmission power control information to the base station. The base station calculates the transmission power from this transmission power control information and adjusts the transmission power.
In such a transmission power control system, for instance, IS-95 standard of TIA/EIA (Telecommunication Industry Association/Electronic Industry Association), a down-link direction transmission control operation is independently performed with respect to an up-link direction transmission control operation.
However, the conventional transmission power control system owns the below-mentioned problem.
Referring now to FIG. 1, the problem of this conventional transmission power control system is explained as follows. FIG. 1 pictorially represents such a condition that a mobile station 300 which is communicated with a base station 200 under normal condition is moved to a shadow portion of a constructive article 700. The constructive article 700 is located at a position where a radio communication channel established between the base station 200 and the mobile station 300 is cut off. It should be noted that this shadow portion of the constructive article 700 corresponds to an insensitive zone where radio waves transmitted from another base station located adjacent to this base station 200 cannot be reached, namely the base station 200 cannot hand off the communication.
Under such a condition, the mobile station 300 operated under the IS-95 standard of TIA/EIA stops the transmission output in such a case that an effective signal of a down-link radio channel cannot be received for a certain time period. At the same time, the base station 200 also cannot receive an up-link radio signal transmitted from the mobile station 300. This up-link radio signal contains quality condition information of the radio channel which is necessarily required so as to execute the transmission power control operation of the down-link radio channel. As a result, the base station 200 stops the transmission power control operation of the down-link radio channel. As a consequence, this conventional transmission power control system owns such a problem that the call drop phenomenon may occur.